Use a custom comparator. : Comparator Interface « Collections « Java Tutorial

1. Java Tutorial
2. Collections
3. Comparator Interface

import java.util.Comparator;
import java.util.TreeSet;

// A reverse comparator for strings.
class MyComp implements Comparator<String> {
  public int compare(String a, String b) {
    return b.compareTo(a);
  }
}

class CompDemo {
  public static void main(String args[]) {
    TreeSet<String> ts = new TreeSet<String>(new MyComp());

    ts.add("C");
    ts.add("A");
    ts.add("B");
    ts.add("E");
    ts.add("F");
    ts.add("D");

    for (String element : ts)
      System.out.print(element + " ");
  }
}